Radio frequency (RF) laterally diffused metal oxide semiconductor (LDMOS) device has gained high mark acceptance. In particular, with the increasingly wide applications of the telecommunications technology, RF LDMOS device, as a new kind of power device, will get more and more attentions.
FIG. 1 depicts the fundamental structure of a conventional RF LDMOS device. It typically includes a heavily doped P-type substrate (with a resistivity of 0.01 Ω·cm to 0.02 Ω·cm). A P-type epitaxial layer is grown on the P-type substrate. The thickness and doping concentration of the P-type epitaxial layer are determined by the specific requirement for a voltage resistance property of the RF LDMOS device (e.g., a voltage resistance of 60 V requires a thickness of 5 μm to 8 μm of the P-type epitaxial layer). Additionally, a P+ sinker is formed through the P-type epitaxial layer and contact with the heavily doped P-type substrate by performing implantation and diffusion processes. The structure further includes a P-well, a gate oxide layer, a gate, an N− drift region, an N+ source region, an N+ drain region, external electrodes (including a source electrode S, a drain electrode D and a gate electrode G), and a metal layer deposited over a backside of the substrate after the substrate is grinded. In the structure of FIG. 1, the source electrode S, or the N+ source region, is connected to the P+ substrate via the P+ sinker.
As the P+ sinker is formed by a diffusion approach, the above described structure suffers from some deficiencies. For example, one deficiency is that lateral diffusion of the P+ sinker increases difficulty in shrinking the area of the device, and another deficiency is that the P+ sinker has a high resistance which affects the performance of the device, especially its working frequency. For this reason, it has been proposed in the prior art to use P+ polysilicon to form the P+ sinker. This approach could help to rectify the above mentioned deficiencies. However, it still has two drawbacks as follows: first, due to limitations in process control, the P+ polysilicon process is not mature and has not been widely used; and secondly, the lateral diffusion issue still exists and the sinker resistance is still much higher than a metal resistance.